(1) Field of the Invention
The present invention relates generally to launch system testing, and more particularly to a method for determining velocity, acceleration and displacement parameters of a projectile being launched from a muzzle or tube of the launch system.
(2) Description of the Prior Art
In order to certify the proper operation of a launch system, such as that used to launch a torpedo from a submarine tube, it is necessary to obtain velocity/time data from the projectile launched by the system. Based on this data, launch velocity, acceleration and displacement parameters can be derived. Current certification procedures for torpedo launch systems rely on a Pressure/Velocity/Displacement (PVD) housing mounted on a breech door of the tube. A wire is attached to the projectile being launched. In the case of a torpedo launch system certification, a shape is launched, which represents the torpedo. When the projectile is launched, the wire is pulled behind the projectile from a reel, which spins a DC generator within the PVD housing. The resulting voltage signal can be read by a computer based data acquisition system and converted to a velocity/time distribution plot. However, the PVD housing provides inherently noisy velocity signals attributed to the gear meshing of the DC generator and the wire payout being subjected to the launch pulse. The noisy data must be filtered in such a way as to obtain representative acceleration and displacement characteristics, which can be compared to launch system specifications to certify the launch system. Additionally, cabling results in a DC shift of the data. However, simply adjusting the PVD data to account for the DC shift and applying standard filtering methods may compromise the calibration of the PVD and cause a phase shift between the displacement and velocity profiles. This may lead to erroneous velocity, displacement, and acceleration time histories and inappropriate interpretation of muzzle and hull exit values.
Accordingly, it is an object of the present invention to provide a method to obtain velocity signals for a launch system, which minimizes the noise components of the signal.
Another object of the present invention is to provide a method to obtain velocity signals for a launch system, which maintains zero phase shifting of the data.
A further object of the present invention is to provide a method to obtain velocity signals for a launch system, which can provide a smooth fit for the velocity/time distribution in order to obtain accurate acceleration and displacement characteristics of the launch system.
Still another object of the present invention is to provide a method to obtain velocity signals for a launch system, which retains the frequency response appropriate for torpedo launched shapes.
Other objects and advantages of the present invention will become more obvious hereinafter in the specification and drawings.
In accordance with the present invention, a method is provided to process velocity data obtained by a PVD housing. The velocity array data is captured by a computer based data acquisition system. Other launch system and test parameters, such as sample rate, muzzle exit length, time of acquisition, decimation factor and low pass cut-off frequency, are provided as inputs for the operation of the method. The method first takes any data less than a predetermined value and assigns to it a value of zero. This serves to eliminate stray, DC cable induced voltages. The method then passes the data and input parameters through a forward/reverse 5th order 6 Hz low pass elliptic filter, resulting in smoothed velocity data. The filter allows 0.005 dB bandpass ripple and a stopband attenuation of 130 dB. This smoothed data is processed to obtain and display an acceleration profile and a displacement profile, as well as to obtain peak acceleration, time at peak, pulse width, muzzle exit velocity and time at exit.
By assigning a value of zero to data below a specified threshold, the method eliminates cable induced voltage components of the signal. The 5th order 6 Hz low pass elliptic filter maintains zero phase shifting of the data and provides a smoothed velocity profile of the data, from which accurate and repeatable acceleration and displacement profiles can be determined. The filter used in the method is chosen to retain the frequency response appropriate for torpedo launched shapes.